Fuel ethanol is currently produced from feedstocks such as corn starch, sugar cane, and sugar beets. However, the production of ethanol from lignocellulose-containing feedstocks, such as agricultural wastes and forestry wastes has received much attention in recent years. An advantage of using these feedstocks is that they are widely available and can be obtained at low cost. Furthermore, a byproduct of the conversion process, known as lignin, can be used as a fuel to power the process instead of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The processing steps for converting lignocellulosic feedstock to ethanol, or other fuels and chemicals, involve breaking down the fibrous lignocellulosic material by a series of chemical and biological treatments to liberate sugar monomers from the feedstock. There are various known methods for producing fermentable sugars from lignocellulosic feedstocks, one of which involves a chemical pretreatment followed by hydrolysis of cellulose with cellulase enzymes and β-glucosidase. The sugars produced in hydrolysis are fermented to a fermentation product in a fermentation carried out subsequent to, or during the hydrolysis of cellulose, using a microorganism such as yeast or bacteria. The fermentation product produced from the lignocellulosic feedstock may be concentrated by any suitable technique. For example, in the production of ethanol, distillation is carried out subsequent to fermentation to recover the ethanol in concentrated form and residual water is subsequently removed by molecular sieves or by other techniques. The residue remaining after concentrating the fermentation product is referred to as “still bottoms” or a “still bottoms stream”.
Although there have been research efforts devoted to producing fuel or other chemicals from lignocellulosic feedstock, the existing technologies have been difficult to commercialize. At present, the cost of producing fuels and other chemicals from lignocellulosic feedstock is still relatively high.
One problem with processes for converting lignocellulosic feedstock to a fuel or other chemical is that the handling and disposal of the still bottoms presents challenges. Disposal of still bottoms is costly, complex and has negative environmental implications. A disposal method that has been proposed for still bottoms derived from cellulosic conversion processes is biological waste water treatment. Another known waste disposal option is incineration, which allows the recovery of heat from the combustion of organics. A further option available for disposal is landfilling the still bottoms. While technically feasible, many of these options for handling and/or disposing of still bottoms require significant capital and operation expenditure. Furthermore, processing the still bottoms to recover inorganic sulfate salts by ion exclusion and using the recovered salts as a fertilizer has also been disclosed, as set forth in U.S. Pat. No. 7,670,813. However, it would be desirable to provide improved or alternate methods for managing still bottoms streams comprising salts arising from processes for producing fermentation products from lignocellulosic feedstocks.